For the purification of raw synthesis gases, which are produced on an industrial scale in gasification plants from coal or/and hydrocarbon feedstocks, for example by reforming with steam or by partial oxidation, and which normally contain some unwanted components, such as water, carbon dioxide (CO2), hydrogen sulfide (H2S) and carbonyl sulfide (COS, physical washing is preferably used. These processes are advantageous, since the raw synthesis gases are presently usually produced at high pressure and the effectiveness of physical washing increases, as a first approximation, linearly with the operating pressure. Methanol washing is of particular relevance for the purification of raw synthesis gases. It makes use of the fact that the solubility coefficients of H2S, COS and CO2 in liquid, cryogenic methanol differ by orders of magnitude from those of hydrogen (H2) and carbon monoxide (CO). Subsequent to the washing process, the methanol is regenerated and returned into the process. Substances such as, for example, hydrogen cyanide (prussic acid, HCN), which is also found in raw synthesis gases, make the regeneration of the methanol washing agent substantially more difficult because they are very strongly bonded to the methanol as a result of their high solubility coefficient. However, hydrogen cyanide remaining and thus concentrating in the methanol is to be avoided, since this poisonous gas is corrosive and, for this reason alone, cannot be tolerated in the products generated from the synthesis gas (CO, H2, oxogas).
According to the prior art, HCN is, for example, washed out with water in a separate wash column which precedes the methanol washing. The loaded washing water, in which the prussic acid is present in strongly diluted form, must be subsequently disposed of in a complex manner or regenerated—e.g. by stripping with air.
Another process makes use of the fact that, compared with CO2, H2S and COS, HCN has a markedly greater solubility in liquid methanol. This makes it possible to wash out HCN from the synthesis gas in a pre-washing step using comparatively small quantities of methanol (Lurgi publication “The Rectisol Process for Gas Purification”). Here, HCN pre-washing is carried out in a first washing step in a methanol wash column, in which sour gases are washed out in a second washing step, using a quantity of methanol that is adjusted to the solubility of HCN. For regeneration, the methanol washing agent loaded with HCN is supplied to the upper region of a stripping column, while methanol washing agent which is only loaded with sour gases is introduced into the lower region of the stripping column. The stripped sour gases and the HCN are subsequently conducted jointly into a Claus plant, where sulfur is obtained from the sour gases and in which the HCN does not disturb.
In order to prevent concentration of HCN in the methanol washing agent to a large extent, the entire methanol washing agent used for separating sour gases and HCN from the raw synthesis gas must be stripped under conditions which are defined by the solubility coefficient of HCN, therefore requiring a large plant-specific and operational outlay. If the stripping is carried out under less demanding conditions, concentration of HCN in the methanol washing agent, with all its negative repercussions, must be accepted.
It is therefore an object of the present invention to provide a process of the generich type, with which the problems of the prior art with respect to the purification of raw synthesis gas containing hydrogen cyanide can be circumvented.
Upon further study of the specification and appended claims, other objects and advantages will become apparent.
According to the invention, such objects are attained by providing a process and associated apparatus in which the loaded washing agent withdrawn from the first washing step is, independently of the remaining quantity of loaded washing agent, subjected to a regeneration step for separating the gas components which have been selectively removed from the raw gas in the first washing step.
The process according to the invention is particularly suitable for regenerating the loaded washing agent from a methanol wash, in which hydrogen cyanide (HCN) is separated from a raw synthesis gas in a first washing step and sour gases (H2S, COS) and/or carbon dioxide (CO2) are separated from said raw synthesis gas in at least one subsequent washing step by means of liquid, cryogenic methanol. The solubility coefficient of hydrogen cyanide in liquid, cryogenic methanol is approximately 100 times greater than that of sour gases or of CO2. For this reason, hydrogen cyanide can be washed out of the raw synthesis gas using a correspondingly lower quantity of methanol. The comparatively small, portion of the methanol washing agent loaded with HCN is supplied to a dedicated regeneration unit, where it is regenerated under the operating conditions necessary for an extensive separation of HCN, while the predominant portion of the methanol washing agent, which is not loaded with HCN but with sour gases and/or CO2 and can therefore be regenerated in a comparatively simple manner, is subjected to a process, preferably a process for regenerating methanol washing agent that is loaded with sour gases and/or CO2, as is sufficiently well known from the prior art.
According to an embodiment of the process according to the invention, the methanol washing agent loaded with HCN is withdrawn from the methanol wash, preferably heated and expanded into a stripping column, in which a gas stream containing HCN is generated by stripping. The stripping gas required for the stripping is suitably obtained by boiling the sump product of the stripping column, which is achieved by supplying it with heat—for example via a steam heated reboiler. A preferred embodiment of the process according to the invention provides that the gas stream containing HCN is cooled down by heat exchange for enrichment and is thereby partially condensed. Sensibly, the HCN containing gas stream is only cooled down until mainly components of the stripping gas are condensed out in the partial condensation.
In the case that the methanol washing agent loaded with HCN contains water, a variant of the process according to the invention provides that the methanol washing agent loaded with HCN and water is introduced into a simple stripping column in order to strip the HCN, with a methanol/water mixture being withdrawn from the sump of said stripping column and this mixture being supplied to a methanol/water separation column. Another variant of the method according to the invention provides that the methanol washing agent loaded with HCN and water is introduced into a stripping column in order to strip the HCN, said stripping column being a divided wall column and being operated in such a way that stripped HCN, water and largely regenerated methanol washing agent are withdrawn separately. In this process variant, no separate methanol/water separation column is required.
To further develop the process according to the invention, it is proposed that the HCN-enriched gas stream withdrawn from the stripping column is disposed of in an environmentally responsible manner. Preferably, for this purpose, the HCN-enriched gas stream withdrawn from the stripping column is fed to a Claus plant and combusted therein.
The invention further relates to apparatus for regenerating the loaded washing agent from a physical gas wash, in which one or more gas components are removed, to a large extent selectively, from a gas mixture (raw gas) to be purified at least in a first of at least two successive washing steps.
With respect to the apparatus, the object of the invention is achieved by the device being provided with a regeneration unit, in which the gas components, which have been selectively removed from the raw gas in the first washing step, may be separated from loaded washing agent and to which solely the loaded washing agent withdrawn from the first washing step may be supplied.
In an advantageous embodiment of the apparatus according to the invention there is provided that, in the regeneration unit, hydrogen cyanide (HCN) may be separated from methanol washing agent loaded with HCN, with the loaded methanol washing agent being withdrawn subsequent to the first washing step of a methanol wash, in which a raw synthesis gas is purified from sour gases (H2S, COS) and/or carbon dioxide (CO2) in at least one further washing step.
A variant of the device according to the invention provides for the regeneration unit to comprise a simple stripping column having a reboiler and overhead condenser, into which the methanol washing agent loaded with HCN may be introduced and from which an HCN-rich gas fraction and a largely HCN-free sump product that consists predominantly of methanol may be withdrawn. If the methanol washing agent loaded with hydrogen cyanide (HCN) contains water, then the sump product of the stripping column consists of a water/methanol mixture, in which case the device according to the invention advantageously comprises a methanol/water separation column, to which the methanol/water mixture may be supplied for separating it into a water fraction and a methanol fraction.
Another variant of the device according to the invention provides for the regeneration unit to comprise a divided wall column that is designed as a stripping column and has a reboiler and overhead condenser, into which methanol washing agent loaded with HCN and water may be introduced and from which an HCN-rich gas fraction, a largely HCN-free methanol fraction and water may be withdrawn separately.